1. Field of the Invention
The present invention relates generally to an acousto-optic analyzer, and more particularly to an acousto-optic spectrometer/polarimeter for performing ultraviolet, visible and infrared imaging spectroscopy; and polarimetry simultaneously and through the same optical aperture.
2. Description of the Related Art
Historically, imaging has been accomplished with photographic film and electro-optical devices. Early examples of these devices were used to identify and record visible broad-band images, i.e., images that may be seen by the human eye. However, more information is contained in a typical scene than can be found through visible broad-band imaging. For example, scenes often contain information that is broad-band, but beyond the range of the human eye. Consequently, imaging devices that identify and record an image from ultraviolet and infrared radiation in a scene were developed.
Also, scenes contain narrow-band spectra information, i.e., electromagnetic spectra arising from either emission or absorption of radiant energy. Narrow-band spectral filters are used to obtain this narrow-band information. For example, hot emission spectra may be used to identify heated elements and molecules by viewing in several narrow-bands so that color is measured. In addition, if an object is semi-transmissive, its makeup may be identified by its absorption spectra using narrow-band spectral filters.
Light that is emitted or reflected from a scene may be directionally polarized. Polarization information may be obtained with a polarization sensor. As the angular orientation of the polarization sensor is changed, the amount of directionally polarized light observed by the polarization sensor also changes, thereby permitting determination of the polarization direction of the directionally polarized light.
All of the above-mentioned types of scene information may be measured with conventional imaging devices. However, the most accurate scene information would be obtained by observing all of the abovementioned types of information simultaneously and through the same optical aperture. This technique would obviate the need for separating spatial errors, temporal errors and system errors before analysis of the image. Although the above-mentioned types of scene information have been obtained simultaneously with conventional imaging devices, they have not been obtained from a common aperture.
One conventional type of optical narrow-band device is the acousto-optic tunable filter (AOTF). In an AOTF, an acoustic wave is transduced in a bi-refringent material by electromagnetic energy and interacts with radiation in the optical domain, i.e., from the ultraviolet region through the infrared region. Broad-band light enters the AOTF in a quasi-collimated beam and is mixed and diffracted in the bi-refringent material by the acoustic wave into two filtered narrow-band light beam and a broad-band light beam. The wavelength of each of the filtered narrow-band light beams depends upon the wavelength of the acoustic wave, i.e., the wavelength of each of the filtered narrow-band light beams is inversely proportional to the wavelength of the acoustic wave. In a conventional AOTF, one of the filtered narrow-band beams is typically used for analysis, while the broad-band light beam and the second narrow-band beam is rejected.